Prior art display devices of this type can comprise a slab waveguide containing three separate diffraction gratings arranged to perform a respective one of three successive functions. A first grating serves as a light input area for diffracting received light in a direction along the slab waveguide. A second grating serves to expand the light from the first grating in a first dimension, and a third grating serves to receive the expanded light and further expand the light in a second dimension orthogonal to the first dimension and to output the result from the waveguide. In order that the second diffraction grating is able to diffract received light in a direction required to enable that diffracted light to reach the third diffraction grating, it is also necessary that the light is received by the second grating from the first grating in an appropriate direction relative to the orientation of the grating lines of the second grating. A misalignment between the orientation of grating lines in the first (input) grating and the second grating will result in a misalignment of light output from the second grating relative to the orientation of the grating lines of the third (output) grating.
This has the overall effect of degrading the quality of the image reproducible from the image-bearing light output by the third (output) grating.
To try to reduce the possibility of misaligned grating lines between first and second diffraction gratings, some prior art methods include pressing or stamping both the first and second gratings as different parts of one general grating structure having one common orientation of grating lines throughout. This may be in the form of two separated grating areas stamped from a common single grating stamper (e.g. FIG. 1A) or in the form of two separate regions of one grating pattern stamped from one stamper (e.g. FIG. 1B).
However, in both cases, in order that the second grating or grating region may receive the input light from the first grating or grating region at an appropriate angle of direction of incidence to the grating lines of the second grating for onward diffraction to the third (output) grating, the light emanating from the first grating or grating region must be re-directed by being reflected back towards the second grating or grating region. A highly reflective surface must be provided along parts of an edge of the slab waveguide in order to achieve this.
The reflective surface must be manufactured to a very high optical standard if it is to be of use. Polishing of a reflective slab edge to a very flat surface is required. This is difficult to achieve and such waveguides are expensive to produce.
The present invention aims to address these matters.